Accustomed as we are to the presence of nuns in the religious landscape of early modern Europe, we imagine a straightforward trajectory by which secular women who entered a convent took vows and donned a veil. This chapter interrogates the seemingly simple process by which laywomen were “converted” into nuns. Upon entering convents, women crossed a border that separated the profane from the sacred. The cloister setting, in turn, required them to adapt to a very different type of existence. They were expected to adhere to monastic principles, many of which were distinctly gendered. Using evidence from English and Spanish convents between 1450 and 1650, this paper will analyze the mechanisms, and the material considerations, that shaped this transformation. How did religious rules, convent architecture, male ecclesiastical oversight, material culture, the rhythms of daily life within the convent, and other factors shape the process by which secular women became nuns? Ultimately, the chapter argues, these conversions were uneven or incomplete. The mechanisms listed above that conditioned this conversion permitted and sometimes even encouraged a complicated identity that blurred the distinction between sacred and secular worlds.